Imaging apparatus as so-called digital still camera and digital video camera, etc., each comprises an imaging device such as CCD or CMOS that can take out projected images as signals, nearly at real times.
In the case that a camera having this kind of the imaging device is employed to photograph stars, etc. at night, it is necessary to perform exposure for a quite long time such as more than 3 minutes.
However, signals driving CCD or the like or a peripheral circuit thereof which may affect charges accumulated in CCD or the like, during exposure period are accumulated as noise, for example.
Here, even a noise which does not matter in the normal photographing for a short exposure time period may be a noise that can not be neglected in reproduction images, in the case that the exposure time period is a long time period as mentioned above, because the noise is also accumulated for a long time.
FIG. 4 is a readout model of frame transfer mode in which readout of a still image recorded in CCD is transferred with 3 fields separately. FIG. 5 is a readout model in monitoring driving mode. And a CCD driving mode at a still image exposure is the monitoring driving mode.
In the case of a long time exposure, accumulation in each pixel is performed corresponding to the exposure time period. After a predetermined accumulating (exposure) time period is elapsed, a mechanical shutter disposed at an optical system or between the optical system and the CCD is dosed to attain a shielding state, and readout operation shown in FIG. 4 is performed.
During a time period before the predetermined exposure time period elapsed (i.e. the exposure operation time period), signals are not output from each pixel of CCD through horizontal and vertical transferring paths. However, since CCD is continued to be operated by 1/30 cycle of the monitoring driving mode, for example if a low-level noise occurs due to the driving signals of the monitoring driving mode, even it is a minor signal which would not result any problem in the normal monitoring, in case that CCD is exposed for 1 second, noise will be accumulated for 30 times, so that the level of the noise signal becomes greater, and causes periodically horizontal-striped patterns with high brightness in the image, and becomes to be extremely observable noise.
Moreover, since in CCD or the like, unforeseen noise is caused by dark-current, and there are variations in light receiving sensitivity among a number of light receiving elements which construct CCD or the like exist, even an exposure is performed by the same light and for the same time period, the differences among signal levels output from each light receiving elements occur.
Furthermore, generation of dark-currents and variations of sensitivity are especially affected by temperature, and the differences mentioned above become greater as the temperature rises.
As shown in FIG. 5, an amplifier disposed at the output end of the CCD horizontal transfer line is operated by driving clocks for CCD. If it keeps working, heat generates certainly, which increases the dark currents of elements adjacent to the amplifier due to the affection of the generated heat, and the differences in the signal levels between them and elements at other portions also become greater, while by lowering a clock frequency at exposure and reducing the number of times of operation, the generation of heat is suppressed and the generation of noise is also inhibited. (Refer to JP-A 3162206 and JP-A 3376193).
Furthermore, techniques of modifying the operation clock of CCD, corresponding to a readout mode of signals data from CCD are proposed. (Refer to JP-A 2003-060994 and JP-A H10-304250).
Moreover, a technology of controlling an exposure time period according to brightness of a subject and controlling the clock of transferring time period to be proportional to the exposure time period is also proposed. (Refer to JP-A 2001-054019).
However, as for recent digital cameras, volume of data that should be read out increased with increase in the number of image pickup devices (higher pixel counts), and at the same time improvement of operation speed is required.